Standard treatment for herpes labialis is conservative, as antiviral medications have not yet been approved by the FDA for treatment of labial herpes. Recently we completed a randomized double-blind placebo- controlled crossover trial to determine whether daily oral acyclovir (an antiviral agent) could be useful in preventing outbreaks of herpes labialis in patients with frequently recurrent disease (6 or more episodes yearly). Twenty patients with proven frequently recurrent labial herpes were randomized to receive acyclovir 400 mg PO BID or matched placebo for 4 months, and then were switched to the opposite therapy for 4 months. Recurrent disease was documented by examination in clinic and by viral culture. Treatment with acyclovir resulted in a 53% reduction in the mean number of clinically determined recurrences mean (+/- SEM) of 1.80 +/- 0.28 episodes in the placebo courses vs. 0.85+0.25 in the acyclovir courses (P<.02) and a 71% reduction in the mean number of virologically documented occurrences (1.40 +/- 0.22 in the placebo courses vs. 0.40 +/- 0.15 in the acyclovir courses) (P<.005) as compared to placebo therapy. We conclude that daily oral acyclovir can significantly reduce the incidence of herpes labialis in immunocompetent adults with proven frequently recurrent disease. This study will be used to hopefully extend FDA approval of acyclovir to include an indication for use in the treatment of herpes labialis.